The subject matter disclosed herein generally relates to air-driven turbine starters. More specifically, the subject disclosure relates to braking mechanisms for air-driven turbine starters.
Aircraft engines, for example, gas turbines, are typically equipped with an air-driven turbine starter mounted on the accessory gearbox, whose purpose is to bring the turbine up to a light-off speed prior to ignition of the engine combustor. The turbine starter is driven by pressurized air provided by an air source such as an auxiliary power unit, another operating engine, or an external air cart connected to the turbine starter. Pressurized air fed into the turbine starter drives is converted to mechanical power causing rotation of a starter output shaft. The starter output shaft transmits this rotation to the drive shaft of the accessory gearbox. Rotation of the gearbox shaft drives the engine which induces airflow into the engine. When the engine rotation reaches a desired speed, combustion is initiated and assists the starter in engine acceleration until the turbine starter is no longer necessary. The flow of pressurized air into the air turbine starter is thus cut off, causing rotation of the starter shaft to slow. As the starter shaft slows, the starter is disengaged via the action of a clutch internal to the turbine starter, for example, a synchronous engagement clutch (SEC) as described in U.S. Pat. No. 5,419,420. The SEC operates by moving clutch elements associated with the starter shaft away from clutch elements associated with the gearbox shaft as the rotation of the starter shaft slows. During the disengagement process, the clutch elements associated with the starter shaft ratchet along the clutch elements associated with the gearbox shaft until the starter shaft slows sufficiently to move the clutch elements associated with the starter shaft a necessary distance to achieve full disengagement. The ratcheting during clutch disengagement contributes to wear of said clutch elements. As such, to reduce wear on the clutch elements and extend their service lives, it is desirable to reduce turbine starter speed quickly so that clutch engagement/disengagement and thus ratcheting time is reduced.